“Black powder” formation in gas transmission lines, such as sales gas lines, is a problem that has had a serious impact on pipeline operations including, but not necessarily limited to, erosion failures of valves, lower efficiency of compressors, clogging of instrumentation and valves, and problems with Health, Safety & Environment (HS&E) compliance. Despite the fact that this has been a persistent problem in the industry, there has been little work towards ascertaining the composition, sources and formation mechanism of black powder in gas transmission lines. Acquiring such knowledge through the development of methodology for measuring the rate of black powder formation is critical for the development of effective strategies to predict and control black powder formation.
“Black powder” is a color descriptive term used loosely to describe blackish material that collects in gas pipelines. It has a “wet” tar-like appearance, or may appear as a “dry” fine powder material that is mainly composed of iron hydroxide, iron carbonate and iron sulfide mixed with various contaminants such as salts, sand and liquid hydrocarbons. J. Smart in “Movement of Black Powder in Gas Pipelines”, Pipeline and Gas Journal, Oct. 1, 2007, notes that black powder is a catch-all term for solids in a gas pipeline, ranging from 100% iron sulfide to 100% iron oxide, also containing rouge, asphaltenes, salt, sand, clay, weld spatter and metallic iron.
In one non-limiting explanation, black powder is believed to be generated during gas production or in gas pipelines when hydrogen sulfide, carbon dioxide or oxygen are present in the gas, by bacterial corrosion of the steel, or from construction when lines are not cleaned adequately. Black powder is even known to exist in new pipelines. In one non-limiting theory of black powder development, water is involved in its formation.
Once in a pipeline, black powder is transported through the pipeline by gas flow. The velocity required to move dry solids in a pipeline can be calculated and depends on pipeline diameter, gas pressure, particle size and particle density. Typical velocities required at 1,000 psi (6.9 MPa) may be 10 ft/sec (3 m/sec) for 8-inch (20 cm) lines, 13 ft/sec (4 m/s) in 24-inch (61 cm) lines and 14 ft/sec (4.3 m/s) in 48-inch (122 cm) lines. Black powder may be a significant operating parameter in wet and dry gas pipelines. Operators report that when black powder moves, it shatters and becomes very small in size, in the range of one micron or less, making it difficult to filter and possibly easier to move.
Deposition of black powder will occur if there are solids in the pipeline fluid and the velocity is not high enough to drag the particles along by viscous flow forces. Sediment deposits can lead to blockage of the line, especially during pigging, while flowing powder can damage compressors, plug filters and damage user equipment. One operator reported that when piping upstream of a compressor was inspected, the piping was half full of black powder, causing shutdown of the compressor and that 60 tons of black powder were subsequently removed from the piping. Black powder also represents a threat to natural gas's reputation as a clean fuel.
It would be desirable if a new methodology for measuring black powder formation rate under field conditions that would allow for the improvements of the current technologies for preventing black powder formation in gas transmission lines could be developed.